Currently, there has been an increasing demand for a process for selective production of propylene because there is a difference in increases of demand for ethylene and propylene and there is an increase in the ethylene supply due to construction of ethane crackers in the Middle East. As a process for producing such propylene, a process is known whereby dimethyl ether (DME) or methanol is fed to a reactor equipped with a zeolite catalyst to obtain a reaction product containing propylene and the propylene is separated from the reaction product.
It is known that, when a reaction product containing propylene is obtained by the reactor mentioned above, a yield of lower olefins such as propylene, etc. can be increased since a ratio of propylene contained in the reaction product can be increased by lowering the partial pressure of dimethyl ether (DME) or methanol occupied in the reactor (Patent Literature 1).
A previously known method involves feeding steam or inert gas into a reactor in order to lower the partial pressure of dimethyl ether (DME) or methanol used as a raw material contained inside the reactor (Patent Literature 2). Particularly, steam is produced in a large amount when dimethyl ether (DME) or methanol is reacted to obtain a reaction product containing propylene, and thus the steam can be conveniently used as a means to lower the partial pressure of dimethyl ether (DME) or methanol contained inside the reactor in view of its convenient supply and low cost.    Patent Literature 1: U.S. Pat. No. 4,083,888    Patent Literature 2: JP-A-2003-535069